1. Field of the Invention
The present invention relates generally to integrated optical (or xe2x80x9cplanarxe2x80x9d) waveguides or components for use in the field of optical communications and information processing, and particularly to an athermalized integrated optical waveguide or component wherein a phased-array type wavelength division multiplexer or demultiplexer (WDM) is employed as a representative example.
2. Technical Background
Optical communications and information processing components which utilize an integrated optical (or xe2x80x9cplanarxe2x80x9d) platformxe2x80x94such as planar wavelength division multiplexers and demultiplexers (WDMs) as a representative examplexe2x80x94require precise control over the optical path difference between adjacent waveguides. The optical path difference is the product of the effective index of refraction of the fundamental mode in the waveguide and the physical path difference between adjacent waveguides. The effective index of refraction of the fundamental mode in the waveguides and the physical path difference between adjacent waveguides are typically both temperature-dependent. In conventional integrated optical WDM components, the medium forming the arrayed waveguides has a noticeable temperature dependency. As a result, temperature variations in the usual operating temperature range (from about 0xc2x0 C. to about 70xc2x0 C.) induce a wavelength shift in centered channel position which is unacceptable in comparison to the typical accuracy requirements (about 0.1 nm) as it may exceed the transmission bandwidth.
Consequently, available WDM optical components of the phased-array type are generally operated in a temperature-controlled environment. Typically, control circuits with heating elements are provided to ensure a stable temperature environment. However, the use of heating elements to achieve active athermalization is undesirable because it increases the overall cost, size, and complexity of the component, and the consumption of considerable power.
In the case of conventional WDMs having a phased-array optical grating comprised of a plurality of silica waveguides and silica cladding, the variation of channel wavelength as a function of temperature predominantly depends on the positive variation of the effective index of refraction of the waveguides with temperature. In an effort to compensate for the positive variation of refractive index as a function of temperature for silica-based materials, polymer overcladding materials having a negative variation of refractive index as a function of temperature have been employed. However, a problem with this arrangement is that, as the temperature varies, the difference in refractive index between the core and the cladding varies, and this may induce temperature-dependent loss. As a result, optical WDM components having a phased-array type grating with a polymer overcladding may not be suitable for use over a wide range of ambient temperatures. Another problem with this optical fiber structure is that the polymer overcladding makes it more difficult to connect optical fibers to the input and output ports of the component.
Another proposed design for maintaining a relatively constant optical path difference between adjacent waveguides in a phased array involves localizing a polymer in a triangular groove in the phased array. The groove is etched in the center of the phased array through the cladding and the waveguides and is filled with a polymer, typically a silicone polymer. The ratio of the optical path difference between adjacent waveguides in the silica region to the optical path difference in the groove can be selected to cancel, or at least minimize, the variation in the mean channel wavelength as a function of temperature. An advantage of the groove design as compared with the overclad design is that the polymer is localized in the middle of the component. This avoids the problem associated with connecting a polymer overcladding component to optical fibers. However, phased-array components having a polymer-filled triangular groove may exhibit a loss of about 2 dB in excess of standard phased-array components. This loss is believed to be attributable to free-space propagation of light into the groove since light propagates freely in the groove, and is only partially collected by the output waveguides of the phased-array component. The estimated loss for such a waveguide increases as a function of the path length in the groove and is not constant, but depends on the number of waveguides in the phasedarray component.
It is useful to identify and isolate two axes of optical free space propagation in the polymer-filled groove of a phased array optical WDM component. These axes include a free-space axis of propagation perpendicular to the waveguides in the array and parallel to the planar substrate (hereafter called the horizontal axis) and a freespace axis of propagation in a direction perpendicular to the waveguides in the array and perpendicular to the planar substrate (hereafter called the vertical axis).
Several solutions have been proposed to reduce losses due to free-space propagation. A first technique which has been proposed is to deconvolute the groove into several grooves having a short free-space length, each groove contributing very small propagation loss such that the total excess loss due to free-space propagation in the grooves is less than about 0.5 dB. Application of this technique is, however, limited because of backreflectance. Current product specifications require less than xe2x88x9245 dB return loss, and it is anticipated that the return loss specification may be less than xe2x88x9255 dB in the future. The combination of multiple reflective interfaces with the fundamental index mismatch that is needed to accomplish athermalization places severe design constraints on the reflection losses. Factors such as providing angled grooves, reflected beam coherence, and the temperature dependence of the reflected beam propagation in these components would make current targets difficult to meet, and could make future targets impossible to achieve. Further, the deep etching process used to make the grooves in glass typically introduces a width error of up to about 1 micrometer. When many narrow grooves are employed, the path length error is amplified so that significant cross talk degradation can result.
Another proposed design for reducing free-space propagation losses in a polymer-filled groove of an athermalized phased-array WDM involves the use of tapered waveguides which collimate the light signal. In this arrangement, each of the waveguides in the phased array are flared at a section immediately adjacent to the polymer-filled groove to achieve an optical loss of about 40 to 50% less free-space propagation loss as compared with a similar component in which each of the waveguides in the phased array has constant cross-sectional dimensions along its entire length. In the flared section of each waveguide, the width of the waveguide core (defined as the waveguide core dimension parallel with the planar substrate and perpendicular to the length direction of the waveguide) gradually increases from a location a short distance away from the polymer-filled groove toward a location abuttingly adjacent to the polymer-filled groove. The flared sections of the waveguides can be formed using conventional etching techniques. However, the thickness (defined as a dimension perpendicular to the planar substrate and perpendicular to the length of the waveguide) of the waveguides is constant along the entire length of the waveguides because conventional techniques for fabricating integrated optical components do not allow precise variations in the thickness of the various layers of material comprising the optical component. As a result, the flared sections of the waveguides can only reduce horizontal free-space propagation losses, and, therefore, can only achieve a maximum loss reduction of 50% since the thickness of the cores of the waveguides cannot be varied to reduce vertical free-space propagation losses.
Another known technique for reducing free-space propagation losses in the polymer-filled groove of an athermalized phased-array WDM component involves etching the vertical walls of the groove with hydrofluoric acid to oblate the original dimensions of the groove. Since the core glass etch-rate is significantly lower than the cladding glass etch-rate, the core protrudes from the groove wall. The component is then processed at a high temperature to melt the protruding core glass to form a convex profile that acts as a lens. The lens as formed is said to collimate the beam and enable greater than 50% loss reduction as compared with an otherwise similar component in which the technique involving ablation etching and melting to form a lens is not employed.
There remains a need for optical WDM components of the phased-array type in which the optical-path difference between adjacent waveguides in the phased-array region can be more precisely controlled to minimize wavelength shifts to an acceptable level while also minimizing power loss in the transmitted signals without the use of active temperature control means, such as heating elements or the expensive processing techniques employed in the prior art.
This invention pertains to a temperature-compensated integrated optical component having a plurality of adjacent waveguides, and to a method for making such a component. The temperature-compensated integrated optical component exhibits very low optical losses that are comparable to those of athermalized components employing a heating element
The method of the present invention can potentially be employed for making athermalized integrated optical components having a slab waveguide disposed in a groove intersecting an array of adjacent waveguides of different lengths, wherein freespace propagation losses in the slab waveguide are negligible (or at least extremely low), and in which reflective losses at core-to-core interfaces are also minimized.
A representative example of a temperature-compensated integrated optical component in accordance with the present invention is disclosed for demonstrative purposes, and includes a planar substrate that either acts as an undercladding or includes an undercladding disposed on the substrate, an optical circuit such as a phased array of adjacent waveguides, and a slab waveguide located in a groove intersecting the phased array. The groove extends to a depth lower than the bottom surface of the waveguides. The phased array includes a plurality of waveguide cores of different lengths supported on the undercladding, with adjacent waveguide cores having a path length difference that is identical for any pair of adjacent waveguides. The groove may be located in a region where the waveguide cores have a straight waveguide segment. The waveguide cores have an index of refraction that increases with increasing temperature. The slab waveguide includes an undercladding layer disposed on the bottom of the groove, a slab core layer disposed over the undercladding layer, and an overcladding layer disposed over the slab core layer. The slab core has an index of refraction that decreases with increasing temperature to compensate for a change in the refractive index of the waveguide cores associated with a temperature change, and thereby maintains a constant optical path difference between adjacent waveguides over the normal operating temperature range.
By forming a slab waveguide having a slab core with planarized upper and lower surfaces aligned with upper and lower surfaces of the plurality of waveguide cores, and by providing an adjacent undercladding and an adjacent overcladding for the slab core, light entering the slab core is guided to eliminate, or at least substantially reduce, vertical free-space propagation losses.
In accordance with another aspect of the invention, the component described above can be modified to further comprise features that limit free-space propagation lengths or collimate light to reduce horizontal free-space propagation losses.
Another aspect of the invention pertains to a method for making a temperature-compensated planar-optical component. The method involves providing an optical component including a substrate that either acts as an undercladding or includes an undercladding layer on the substrate, forming a plurality of waveguide cores of different lengths on the undercladding, and disposing an overcladding over the waveguide cores. Adjacent waveguide cores may have a path length difference that is identical for any pair of adjacent waveguides. The component includes a groove intersecting the waveguides that extends to a depth lower than the bottom surface of the waveguides, and divides each of the waveguides into a first segment and a second segment separated by a free space. In the example of a phased array of waveguides, the waveguides may be generally curved, and the groove may be formed in a region of the waveguides this is generally straight and have a substantially triangular boundary. Next, an undercladding is formed on the bottom of the groove. The undercladding may be provided with a planar upper surface located in a plane generally coincident with the lower surfaces of the waveguide cores. A slab core is then formed within the groove above the undercladding. The slab core may also be provided with a planar upper surface located in a plane generally coincident with the upper surfaces of the waveguide cores. Thereafter, an overcladding layer is formed over the slab core.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows, together with the claims and appended drawings.
It is to be understood that the foregoing description is exemplary of the invention only, and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute part of the specification. The drawings illustrate various features and embodiments of the invention which, together with their description, serve to explain the principles and operation of the invention.